Overall objectives of the total project: (1) To develop new methods for the treatment of cholesterol cholelithiasis by investigating the efficacy of ursodeoxycholic acid in the dissolution of gallstones in vivo. (2) To study the toxicity of ursodeoxycholic acid in comparison to that of chenodeoxycholic acid in rhesus monkeys. (3) To characterize the mechanism of the therapeutic action of ursodeoxycholic acid in comparison to that of chenodeoxycholic acid by studying the effect of these two bile acids on bile acid metabolism, and the solubility of cholesterol in bile. (4) To investigate the source and site of the increase in the formation of ursodeoxycholic acid, which is found in some patients treated with chenodeoxycholic acid. (5) To study the role of bacterial alterations of bile acid metabolism in the pathogenesis of cholesterol cholelithiasis on the disease model of ileitis. (6) To determine the incidence and type of disturbances of bile acid metabolism in ileitis. (7) To study the determinants of the diarrheal effect of bile acids in different conditions of ileal dysfunction and bile acid malabsorption.